1. Field of the Invention
This invention relates to a scanning exposure unit, a semiconductor laser driving circuit and an image forming apparatus, and more particularly to a scanning exposure unit adapted to turn on a semiconductor laser on the basis of image information and scan and expose a photosensitive body with a laser beam outputted from a semiconductor laser; a semiconductor laser driving circuit; and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus of an electrostatic system utilizing a laser, including a laser printer is being spread at present. In this image forming apparatus utilizing a laser (mainly, a semiconductor laser, which will hereinafter be referred to as xe2x80x9cLDxe2x80x9d), a scanning exposure unit adapted to scan and expose a photosensitive body with a laser beam is used.
To be more exact, an electrostatic latent image is formed on a photosensitive body a surface of which is uniformly charged with the scanning exposure unit, by scanning the same surface with a laser beam modulated on the basis of image data. After this electrostatic latent image has been developed with a toner supplied thereto, transfer paper is superposed on a developed toner image, and the toner image is transferred onto the transfer paper by having the toner electrostatically adsorbed to an outer surface of the transfer paper. The transferred toner image is then fixed by applying heat or a pressure to the transfer paper, to form an image.
A LD driving method used in the scanning exposure unit will now be described. In the case where an electrostatic latent image is formed by using a laser beam, the condition of formation of an electrostatic latent image differs with an optical intensity (quantity) of the laser beam, so that it is necessary to drive the LD so as to obtain a laser beam of a predetermined optical intensity (a predetermined quantity of output light). As shown in FIG. 12, the LD has the characteristics of outputting coherent light when a driving current therefor has attained a predetermined level (which will hereinafter be referred to as xe2x80x9cthreshold currentxe2x80x9d) Ith.
In order to form an electrostatic latent image on a photosensitive body, it is necessary to output a laser beam modulated on the basis of an image signal (video signal) based on image data, i.e., on the basis of an ON/OFF signal indicating the turning on and off of the LD.
Therefore, as shown in FIG. 13, a LD driving circuit 400 was provided with a current source 404 for supplying a current corresponding to a desired intensity of light to a LD402, and a switching circuit 406 for modulating a laser beam outputted from LD402 on the basis of image data. The current source 404 is adapted to supply an electric current a level of which corresponds to that of a set voltage to LD402 through the switching circuit 406, and the switching circuit 406 is adapted to supply the electric current to and stop the electric current flowing to LD402, on the basis of the video signal. This enables a laser beam having a desired optical intensity and modulated on the basis of image data to be outputted. Such a modulation method is generally called a pulse width modulation (PWM) system.
The modulation methods positively utilizing the fact that the condition of formation of a latent image differs with a quantity of output light of a laser include a pulse amplitude modulation (PAM) system. In the formation of an image by this PAM system, an image is formed by varying an emission intensity (i.e. a quantity of output light) of LD on the basis of the image data.
Japanese Patent Laid-Open No. 206366/1989 discloses the techniques concerning the driving of LD by the PAM system. According to the techniques, some of plural LD driving current sources are selected on the basis of an intensity setting signal from the outside, and the sum of the currents from the selected current sources is supplied to the LD, whereby the intensity of light (quantity of output light) of LD can be varied. In general, when the optical intensity increases, the range in which a latent image is formed increases, and a dot image developed becomes large as compared with that developed when the optical intensity is low.
In an image forming apparatus, an image besides ordinary letters is printed in some cases, and it is known that the quality of an image, especially, the reproducibility of an intermediate color (which will hereinafter be referred to as xe2x80x9chalftonexe2x80x9d) receives the influence of transitional characteristics of LD being modulated.
For example, Japanese Patent Laid-Open No. 2051:83/1989 discloses that, when LD is turned on and off by a modulation signal (pulse signal), fluctuation occurs in an optical output in a transitional period, which causes nonuniformity of the density of an image formed, failure in the formation of a beautiful (rectangular) pulse waveform corresponding to the modulation signal, and failure in the faithful reproduction of a halftone of the image. Japanese Patent Laid-Open No 2878/1991 discloses that, when distortion occurs in a driving current of LD, a waveform of an optical output from the LD is also distorted to cause turbulence to occur in a dot image formed, and therefore a decrease in the quality of the image.
FIG. 14 shows waveforms of an optical output from LD. Referring to FIG. 14, a curve {circle around (1)} shows an ideal rectangular waveform of optical output, and a curve {circle around (2)} shows a waveform of optical output having a rounded rising edge, and a curve {circle around (2)} shows a waveform of optical output having a rippled rising edge. FIG. 15 shows the relation (output characteristics), which corresponds to each of the output waveforms of FIG. 14, between a pulse width and an average quantity of optical output in the PWM system.
In FIG. 15, a line {circle around (1)} shows ideal output characteristics corresponding to the waveform of optical output of the curve {circle around (1)} in FIG. 14 and generally called output characteristics having a lineality. Namely, an average quantity of optical output increases or decreases in proportion to an increase and a decrease in the duty of a pulse width.
In the case of the waveform (the curve {circle around (2)} in FIG. 14) having a rounded rising edge, the characteristics of an optical output become as shown the curve {circle around (2)} in FIG. 15. In this case, when the duty is large, the output characteristics do not differ greatly from those in the ideal case of the line {circle around (1)} in FIG. 15 but, when the duty is decreased, the average quantity of output light drops suddenly by a level corresponding to the rounded portion of the waveform. As a result, a minute image displayed by reducing the duty disappears and cannot be reproduced, and a low density (highlight) of a halftone becomes lower than a regular level and unable to be reproduced.
In the case of the waveform (the curve {circle around (3)} in FIG. 14) having a rippled rising edge, the characteristics of an optical output become as shown the curve {circle around (3)} in FIG. 15. In this case, when the duty is large, the output characteristics do not differ greatly, either, from those in the ideal case of the line {circle around (1)} in FIG. 15 but, when the duty is reduced, an average quantity of optical output becomes large contrariwise as compared with that of optical output in the case of the curve {circle around (2)} in FIG. 14. As a result, a minute image is crushed and cannot be reproduced, and the highlight becomes denser than a regular level and unable to be reproduced.
For these reasons, it has been necessary to set a waveform of an optical output being modulated to a turbulence-free rectangular waveform identical with a waveform of a modulation signal. To meet the requirements, the waveform of an optical output being modulated is corrected by using the differentiation circuit disclosed in Japanese Patent Laid-Open No. 205183/1989, or by using a more regular snubber circuit (refer to FIG. 16).
With the development of the digitization techniques and image processing techniques in recent years, the improving of the resolution of an image forming apparatus has been advanced increasingly, and a demand for attaining a resolution (corresponding to a genuine write density and not to a write density obtained by a correction process carried out in the fast scanning direction, in other words, corresponding to a write density in the slow scanning direction) for forming an image by dots the diameter of which is smaller than that of a laser beam from a scanning exposure apparatus has increased.
FIG. 17 shows the relation between the diameter of a laser beam from a regular scanning exposure unit using a 780 nm LD and a resolution thereof. The diameter of a laser beam is generally defined as a diameter of a point in which the optical intensity becomes 1/e2 (wherein e represents a bottom of a natural logarithm) in the center of the laser beam.
As shown in FIG. 17, a dot size (size of one dot) is determined as about 42 nm for obtaining a resolution of 600 dpi (dot per inch), and about 21 nm for obtaining a resolution of 1200 dpi, with respect to a diameter of 60-80 nm of a laser beam from a regular scanning exposure unit. Namely, even at a leading resolution at present of 600 dpi, a dot size has already become smaller than a diameter of a laser beam.
According to a related art scanning exposure unit, when a dot size is thus smaller than a diameter of a laser beam, a ratio (which will hereinafter be referred to as xe2x80x9cheight and width ratioxe2x80x9d) of a thickness of a scanning line in a fast scanning direction to that of scanning line in a slow scanning direction, and the reproducibility of one dot are deteriorated greatly, and the quality of an image lowers.
The height and width ratio will now be described in detail. An ideal value (normal value) of the height and width ratio is 1, i.e., the condition in which, even when lines of the same number of dots are drawn both in a longitudinal direction (slow scanning direction), and in a lateral direction (fast scanning direction) as shown in FIG. 18A, the thickness of the lines becomes equal is an ideal (normal) condition.
In general, in the formation of an image by the PWM system, the thickness of a line (lateral line) drawn in the lateral direction depends mainly upon a diameter Ds (refer to FIG. 18C) in the slow scanning direction of a laser beam. The thickness of a line (longitudinal line) drawn in the longitudinal direction depends mainly upon the lighting time of LD, the lighting time for one dot decreasing with an increase in the resolution.
Therefore, as shown in FIG. 18B, when the dot size is smaller than the diameter of the laser beam, the lateral line remains to have a thickness determined by the diameter Ds of the laser beam in the slow scanning direction, which thickness is larger than the dot size for attaining a desired resolution, while the longitudinal line conversely becomes thin.
To be more exact, during the development of an electrostatic latent image, more toner is deposited on a portion of a higher exposure rate of a photosensitive body, and less toner on a portion of a lower exposure rate thereof. When a photosensitive body is subjected to scanning exposure with a laser beam, with LD driven by an ideal waveform of an optical output, the exposure rate of the portion of the photosensitive body which corresponds to a position in the vicinity of a LD lighting starting position becomes lower than a predetermined exposure rate as shown in FIG. 19. When the exposure rate is low, a latent image formed on the photosensitive body becomes shallow, and the quantity of the toner deposited on the photosensitive body becomes lower (hatched portion) than a level required to print the image, to cause the so-called omission of image to occur. Consequently, the longitudinal line formed by turning on the LD for a short period of time becomes thin because of an increased resolution.
When the LD lighting time is lengthened, the longitudinal line can be thickened. Since the one dot lighting time is determined naturally on the basis of the resolution and in view of the optical designing of the image forming apparatus, the LD lighting time cannot be lengthened thoughtlessly even when the obtaining of a thicker longitudinal line is desired. When the optical intensity (quantity of optical output) of LD is reduced, the lateral line can be thinned but the longitudinal line is also thinned correspondingly.
After all, when the resolution increases, the lighting time for one dot determined in view of the optical designing of the apparatus becomes short and the longitudinal line becomes thinner, so that the height and width ratio keeps lowering. The shape of one dot in the toner image tends to have a larger longitudinal size due to an increase in the resolution, and the reproducibility of one dot decreases.
The deterioration of the height and width ratio and reproducibility of one dot also exert influence upon the reproducibility of a halftone. For example, when the area gradation of a dither system is utilized, the density of image is expressed by the number of pixels smeared away with dots in one screen cell formed of plural longitudinal and lateral pixels (sub-pixels) as shown in FIG. 20A, i.e. the area of a portion smeared away of one screen cell.
However, when one dot is elongated longitudinally as mentioned above, the pixels cannot be smeared away successfully as shown in FIG. 20B, and the area of a portion smeared away of one screen decreases, so that the reproducibility of highlight is deteriorated. When longitudinally adjacent dots overlap each other as shown in FIG. 20C, to cause the smoothness of density of an image to get out of order (so-called tone jump), and the dots to be arranged laterally, the density increases. In the case where a multiline screen is used, the reproducibility of highlight is also impaired for the same reasons.
Solving these problems by the PAM system is also conceivable. However, an increase in the operation time is demanded in proportion to the level of the resolution, and more number of current sources are required to solve the problem of setting a very low level of optical intensity. Therefore, the construction of a driving circuit becomes complicated, and the power consumption increases, so that this system is disadvantageous in view of the cost.
The reducing of the diameter of the laser beam is also conceivable but the diameter of the laser beam cannot be changed easily in a 780 nm LD, which is used generally at present, in view of the characteristics and cost thereof. Even when a LD of a wavelength shorter than 780 nm has become usable, it will encounter the same problems before long when the resolution has been further improved.
At present, these problems are made inconspicuous by changing the processing conditions for electrophotographs but it is easily anticipated that, when the resolution improving techniques have been further advanced in the future, solving the problems by merely changing the processing conditions will become difficult and a substantial solution must be considered.
The present invention has been made in view of the above circumstances, and provides a scanning exposure unit capable of forming at a low cost an image of a high quality with a resolution at which a dot size is smaller than a diameter of a laser beam, a semiconductor laser beam driving circuit and an image forming apparatus.
According to an aspect of the present invention, the scanning exposure unit, adapted to turn on a semiconductor laser on the basis of image information and subject a photosensitive body to scanning exposure with a laser beam outputted from the semiconductor laser, is provided. The optical intensity at a rising edge of the laser beam is increased to a level higher than that of a steady-state optical intensity at every laser beam lighting time.
With this aspect, the photosensitive body is exposed at an optical intensity higher than a steady-state optical intensity at a rising edge of the laser beam at every laser beam lighting time. This enables an exposure rate of the portion, on which only a shallow and narrow latent image can be formed by a related art scanning exposure unit, of the photosensitive body which corresponds to a position in the vicinity of a semiconductor laser lighting starting position to be increased, and a deep and wide latent image to be formed. Consequently, a height and width ratio and the reproducibility of one dot and a halftone at a resolution (dot size) smaller than the diameter of a laser beam can be improved, and an image of a high quality can be obtained.
In the above-described aspect, it is recommended that the optical intensity at a rising edge of the laser beam be increased by generating overshoot in an optical output from the semiconductor laser.
According to another aspect of the invention, the semiconductor laser driving circuit adapted to control the driving of a semiconductor laser used as a light source when a surface of a photosensitive body is scanned with and exposed to a laser beam has an optical output control circuit for setting the level of an optical output higher than that of a steady-state optical output and controlling the optical output when the lighting of the semiconductor laser is started.
With this aspect, an optical output the level of which is higher than that of a steady-state optical output can be obtained by an optical output control circuit when the lighting of the semiconductor laser is started. The time and the quantity of light for and at which the optical output, i.e. the optical output the level of which is higher than that of a steady-state optical output are maintained can also be controlled. This enables an exposure rate of the portion, on which only a shallow and narrow latent image can be formed by a related art apparatus, of the photosensitive body which corresponds to a position in the vicinity of a semiconductor laser lighting starting position to be increased, and a deep and wide latent image to be formed.
It is recommended that this optical output control circuit be formed of an overshooting circuit for generating overshoot in an optical output from the semiconductor laser as described in a fourth aspect of the invention.
It is also recommended that the overshooting circuit includes at least one of a resistor, an inductor and a capacitor (hereinafter referred to as R, L, and C), and is expressed by a linear differential equation of at least second order.
It is further recommended that the shape of overshoot generated by the overshooting circuit be optimized by setting the values of at least one of R, L, C on the basis of at least one of a writing density, a writing speed, and diameters of the beam spot in the fast and slow scanning directions.
According to a further aspect of the invention, the image forming apparatus has a scanning exposure unit to turn on a semiconductor laser by the semiconductor laser driving circuit of any of the above aspects, and on the basis of image information, subject a photosensitive body to scanning exposure by a laser beam, in which a level of an optical intensity at a rising edge thereof at every laser lighting time is higher than that of a steady-state optical intensity thereof, outputted from the semiconductor laser.
This enables an optical intensity higher than a steady-state optical intensity to be obtained, and a photosensitive body to be exposed at an optical intensity higher than a steady-state optical intensity, at a rising edge of the laser beam.
Namely, an exposure rate of the photosensitive body at a point in time close to a semiconductor laser lighting time, at which nothing but a shallow and narrow latent image can be formed by a related art scanning exposure unit, can be increased, and a deep and wide latent image can be formed. This enables a height and width ratio, and the reproducibility of one dot and a halftone at a resolution (dot size) smaller than the diameter of a laser beam to be improved, and a high-quality image to be obtained.
According to still another aspect of the invention, the image forming apparatus includes an exposure member for forming an electrostatic latent image on a photosensitive body by a laser beam, a developing member for electrostatically developing the electrostatic latent image with a toner and thereby forming a toner image on the photosensitive body, and a transfer member for transferring the toner image onto a transfer medium and thereby forming an image thereon, in which the exposure member has an optical output control member for increasing an optical intensity at a rising edge of the laser beam to a level higher than that of a steady-state optical intensity at every laser lighting time during the formation of the electrostatic latent image.
With this aspect, the optical intensity at a rising edge of the laser beam is controlled by the optical output control member at every laser lighting time so that this intensity increases to a level higher than that of a steady-state optical intensity. Accordingly, the exposure member is capable of exposing the photosensitive body with an optical intensity higher than a steady-state optical intensity at a laser lighting starting time, increasing to a high level an exposure rate of the photosensitive body at a point in time close to the laser lighting starting time at which nothing but a shallow and narrow latent image can be formed by a related art apparatus of this kind, and forming a deep and wide latent image. This enables a height and width ratio and the reproducibility of one dot and halftone in a case where a resolution (dot size) is smaller than the diameter of the laser beam to be improved, and a high-quality image to be obtained.
In this image forming operation, it is recommended that, as described in a ninth invention, the optical intensity at a rising edge of the laser beam converges at a point of not larger than 60%, preferably substantially 35% of the irradiation time for forming one pixel, and that a maximum value of this optical intensity be not lower than 1.1 times and not higher than 1.7 times, preferably substantially 1.4 times a value of a steady-state optical intensity.
It is also recommended that the laser beam be focused as a beam spot on an outer surface of the photosensitive body by an imaging forming device with an outer surface of the photosensitive body scanned relatively therewith to form an electrostatic image thereon, and that a diameter of the beam spot in the fast scanning direction be set larger than that in the slow scanning. Especially, it is recommended that the diameter of the beam spot in the fast scanning direction be set not smaller than 1.25 times that in the slow scanning direction thereof.
It is further recommended that the image forming apparatus has an environmental information obtaining member for obtaining environmental information including at least one of temperature and humidity, and a steady-state optical intensity control device for changing the steady-state optical intensity on the basis of the environmental information obtained by the environmental information obtaining member.